generic-library/bionic
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity

mmap: fix calculation of is_private_anonymous variable

Browse Source 
Currently is_private_anonymous is calculated as true if _either_
MAP_PRIVATE or MAP_ANONYMOUS is set, which is a mistake.
According to Documentation/vm/ksm.txt, "KSM only merges anonymous
(private) pages, never pagecache (file) pages". MAP_PRIVATE can
still be set on file cache pages so in order to not redundantly
set MADV_MERGEABLE on pages that are not fitted for it, both
MAP_PRIVATE and MAP_ANONYMOUS should be set.

Along with this fix, add an extra check that the mapped page is
not a stack page before setting MADV_MERGEABLE for it. Stack pages
change too quickly and always end up in KSM 'page_volatile' list.

Change-Id: If4954142852f17cc61f02985ea1cb625a7f3dec6
This commit is contained in:
Vitaly Vul 2015-09-24 14:28:59 +02:00 committed by Elliott Hughes
parent f30170dfa5
commit ee67dd75e2
1 changed files with 6 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
libc/bionic/mmap.cpp
View File

@ -54,10 +54,14 @@ void* mmap64(void* addr, size_t size, int prot, int flags, int fd, off64_t offse
return MAP_FAILED;
}
bool is_private_anonymous = (flags & (MAP_PRIVATE | MAP_ANONYMOUS)) != 0;
bool is_private_anonymous =
(flags & (MAP_PRIVATE | MAP_ANONYMOUS)) == (MAP_PRIVATE | MAP_ANONYMOUS);
bool is_stack_or_grows_down = (flags & (MAP_STACK | MAP_GROWSDOWN)) != 0;
void* result = __mmap2(addr, size, prot, flags, fd, offset >> MMAP2_SHIFT);
if (result != MAP_FAILED && kernel_has_MADV_MERGEABLE && is_private_anonymous) {
if (result != MAP_FAILED && kernel_has_MADV_MERGEABLE &&
is_private_anonymous && !is_stack_or_grows_down) {
ErrnoRestorer errno_restorer;
int rc = madvise(result, size, MADV_MERGEABLE);
if (rc == -1 && errno == EINVAL) {